Backspacing vs Offset: Conversion, Formula and When to Use Each

Backspacing is the distance in inches from the wheel's mounting face to the inner edge of the wheel — the lip that faces the suspension once bolted up. Offset, marked ET, is the distance in millimetres from the mounting face to the wheel's centerline. They describe the same piece of geometry from different reference points.

The conversion formula is simple once units are aligned: backspacing (in) = (wheel width / 2) + (offset mm / 25.4) + 0.5. The half-inch accounts for the lip beyond the bead seat (wheel widths are quoted bead-to-bead, not flange-to-flange). To go the other way: offset (mm) = (backspacing − wheel width / 2 − 0.5) × 25.4.

Example: an 8.5″ wide wheel with ET35 has backspacing of (8.5/2) + (35/25.4) + 0.5 ≈ 6.13″. Drop to ET20 and backspacing falls to about 5.54″ — the wheel moves outward by roughly 15 mm of poke.

Convention varies by region. European OEMs and most modern factory specs use ET, because the value is stamped on the back of every wheel during manufacturing. North American aftermarket — particularly trucks, off-road and classic muscle — historically uses backspacing because it directly answers 'how much room behind the wheel?'.

Neither measurement is more accurate. Both describe the same mounting plane. What changes is which question they answer first: ET answers 'where is the wheel sitting laterally relative to the hub?', backspacing answers 'how much suspension clearance do I have?'.

When buying wheels, use whichever your supplier publishes — but always convert and cross-check. A common fitment mistake is reading backspacing on one wheel, ET on another, and assuming they sit the same. Run both through the conversion formula before ordering.

Backspacing changes faster than ET in absolute terms because it folds in half the wheel width. A 1-inch wider wheel with the same ET has 0.5″ more backspacing, even though offset is identical. That is why wider wheels with stock ET often hit the strut before they hit the fender.

Use the wheel offset calculator to enter width and ET; the result tells you exactly how inner clearance (the backspacing-equivalent dimension) changes versus your current wheel. No mental conversion needed.